Flotation device for offshore platform assembly



Feb. 6, 1968 c. A. RYBICKI 3,367,119

FLOTATION DEVICE FOR OFFSHORE PLATFORM ASSEMBLY Filed Jan. 20, 1966 4Sheets-Sheet 1 F G 2 INVENTOR.

CHESTER A. RYBICKI A TTORNEYS WM Q 1968 c. A. RYBICKI 3,

FLOTATION DEVICE FOR OFFSHORE PLATFORM ASSEMBLY iled Jan. 20, 1966 4Sheets-Sheet 2 lIIIIIII [III \lllllnll\i\llllilll\i\lil INVENTOR.

CHESTER A. RYBICKI FIG 3 BY ATTORNEYS Feb. 6, 1968 c. A. RYBICKI3,367,119

FLOTATION DEVICE FOR OFF-SHORE PLATFORM ASSEMBLY Filed Jan. 20, 1966 4Sheets-Sheet 5 INVENTOR.

CHESTER A RYBICKI f iaz ATTORNEYS Feb. 6, 1968 c. A. RYBICKI 3,367,119

FLOTATION DEVICE FOR OFFSHORE PLATFORM ASSEMBLY Filed Jan. 20, 1966 4Sheets-Shet 4 United States Patent 3,367,119 FLOTATION DEVICE FOROFFSHORE PLATFORM ASSEMBLY Chester A. Rybiclti, Houston, Tex., assignorto Signal Oil and Gas Company, Los Angeles, Calif. Filed Jan. 20, 1966,Ser. No. 528,329 7 Claims. (Cl. 61-465) ABSTRACT OF THE DISCLOSURE Anoffshore platform assembly comprises a platform and a plurality ofdepending legs, means connected to the platform and the legs toadjustably position the legs vertically and laterally relative to theplatform, wherein each of the legs comprises a rigid frame structurehaving a submersible lower portion and wherein at least one of the legsincludes a closed chamber disposed in the submersible portion of theframe with means for opening the chamber to allow the entry of water andan adjustable standpipe communicating with the exterior of the chamberand extending to a predetermined level within the chamber whereby theamount of air and water within the chamber may be automatically andadjustably controlled.

The present invention generally relates to offshore platform assemblieswhich are adapted to be erected in a body of water, and moreparticularly relates to portable offshore platform assemblies havingadjustable legs which can be lowered to erect and support the platformthereof over a body of water and which can be raised for transit of theplatform.

Various conventional offshore platforms heretofore have been providedfor such purposes as offshore oil drilling operations, mining operationsand the like. Such platforms usually are towed to a desired location bya barge, after which they are secured in place, as by legs, anchors,etc. In some instances, the platforms are supported above the waterlevel by a plurality of legs. The legs are connected to the platform andset into the floor beneath the body of water, after which the platformis jacked up or otherwise raised to a level above the surface of thewater, whereupon an operation such as offshore drilling is commenced.When it is desired to relocate the platform, the platform is loweredinto the water, the legs are withdrawn from the floor and raised, andthe platform and legs are towed to a new location.

One of the most difficult problems encountered in erecting an offshoreplatform assembly is that of lowering the legs and properly positioningthem before embedding them into the ocean, lake or sea floor. The legsare unually extremely long, for example, several hundred feet long,because of the substantial depths of water which are frequentlyencountered at locations Where the platform is to be erected. Also, thelegs are usually relatively heavy because of the heavy-duty constructionnecessary to support the platform and all of the equipment on it, suchas a derrick in the case of oil drilling operations. Strong currents andtides and occasional severe storms place additional strengthrequirements on the legs. The heavy legs incur dead-weight moments whichfrequently result in damage to the legs, such as buckling, and/or damageto the means connecting the legs to the platform. Such damage results inconsiderable loss of time and money and may necessitate towing theplatform to shore in order to effect necessary repairs.

Because of the relatively great length of the legs with respect to theoverall dimensions of the platform, the legs are often positionedoutward from the platform at a small angle from the vertical to providea larger foundation area for the erected platform.

It is therefore an object of this invention to provide an improvedoffshore assembly which includes improved supporting legs.

It is a further object of this invention to provide an improved offshoreassembly which includes one or more legs adapted to minimize undesirableblending forces exertable on the leg during installation of theassembly.

It is a still further object of this invention to provide an improveddrilling rig platform assembly which incorporates a plurality of legsadapted to reduce the apparent weight thereof and stresses thereonduring installation of the assembly.

The foregoing and other objects are accomplished, in accordance with theinvention, by providing an improved platform assembly which includes aplurality of depending legs adjustably mounted to a platform in a mannerso that they can be lowered into a body of water and can be positionedvertically and laterally. At least one of the legs and preferably all ofthe legs include improved components adapted to reduce the apparentweight thereof and minimize the bending forces.

The improved components of the leg(s) comprise a closed container, theinterior of which defines a closed, water-tight chamber. The containeris mounted adjacent the lower end of the leg and is adapted to providethe leg with a sufficient amount of buoyancy to reduce, at leastpartially, the effect of bending forces created by the deadweight of theleg as it is lowered in the water. For this purpose, the container isequipped with means for admitting Water to and expelling water from thechamber to a predetermined level.

As a specific example, an offshore drilling rig platform assembly hasbeen provided for oil drilling operations in the North Sea. The assemblyincludes a horizontal platform which is massive, in generally hexagonalconfiguration, with an inset docking area. Its major diameter is 175feet, and its minor diameter is 164 feet. At three approximately equallyspaced points on the exterior of the platform are pivotal-1y connected,adjustable, vertically extending legs, each of which has an overalllength of about 400 feet and a weight of about 1,750,000 pounds, ofwhich 500,000 pounds is a flotation can, and the remainder is structuraltubing, etc. Each leg is generally equilaterally triangular incross-section, with a length per side of about 38 feet, and with theflotation can disposed at the lower end thereof and configured toprovide a wedge-shaped lower terminus for embedding the leg in the seafloor. The can has an overall length of about feet and is of suitablecross-section to conform generally to the size and shape of the leg.

The can is provided with a standpipe utilized in combination with othercomponents to establish, upon demand, a predetermined volume of airwithin the can. The standpipe is mounted in the can with an upperportion extending outside the can and a lower portion within the can andadjacent the vertical side of the can which is closest to the platformso as to maximize the air volume in the can when the portion of the legbelow the platform is inclined away from the platform at an angle fromthe vertical. A pair of valve and air lines are provided so that thewater level in the can can be established at the bottom end of the lowerportion of the standpipe. By passing air below this level to the outsideof the can, the standpipe serves to define a maximum volume of air whichmay be established within the can. By positioning the standpipe close tothe wall of the leg frame adjacent the platform, the volume of air whichcan be stored in the can varies, depending upon the amount ofinclination of the leg, a greater inclination of the leg permitting agreater volume of air to be stored in the can, thereby automaticallycompensating for increased bending moment and stress which is likely toresult from the increased inclination of the leg during lowering of theleg.

It will also be understood that the maximum volume of water which may beestablished in the can can also be regulated by making the standpipevertically and/or horizontally adjustable. Thus, it can be positioned ata preselected level and/or distance from the side of the can closest tothe platform. Alternatively, levelsensing devices can be located atvarious levels within the can. Such level-sensing devices can beconnected to a servo system which adjusts the pressure of the air supplyto establish and maintain the water at a chosen level. In this manner bythe buoyancy provided by the can may be adjusted for various depths andleg angle conditions, etc.

Further objects and advantages of the invention will be apparent from astudy of the following detailed description and the accompanyingdrawings, of which:

FIGURE 1 is a schematic top plan view of an offshore platform assemblyin accordance with the invention showing three legs adjustably connectedto the platform;

FIGURE 2 is a detailed schematic top plan view of a leg of FIG. 1;

FIGURE 3 is a partial section of the leg of FIG. 2 taken along thesection line 3-3 of FIG. 2;

FIGURE 4 is a schematic side elevation of the platform of FIG. 1 showingthe legs in a raised position;

FIGURE 5 is a schematic side elevation showing a portion of the platformand a leg in a lowered position; and

FIGURE 6 is a schematic side elevation showing the legs embedded in thefloor beneath a body of water.

Referring, more particularly, to FIG. 1 an ofifshore platform assembly 9having three generally vertically extending legs 11 is schematicallyillustrated. The legs 11 are mounted on a generally horizontal extendingplatform 13, the top surface of which may be used to support equipmentsuch as a drilling derrick. It will be understood that the platform 13may be supported by any number of legs 11 and that the legs 11 areusually generally uniformly spaced apart and disposed adjacent theperiphery of the platform 13. The platform 13 and legs 11 may be of anysuitable size and shape and construction. In FIG. 1, the platform 13,schematically illustrated, is generally hexagonal in shape with each ofthe legs 11 generally triangular in cross-sectionand located at a givenside of the platform and positioned, relative to one another, at thecorner of an equilateral triangle (shown partially in dotted outline).With the lower part of each of the legs 11 inclined outwardly from theplatform 13 when in an installed position, the symmetrical arrangementof FIG. 1 affords considerable rigidity and resistance in a number ofdirections against bending forces.

FIGS. 2 and 3 schematically show details of one of the legs 11 ofFIG. 1. Thus, the leg 11, as shown in FIGS. 2 and 3, comprises a rigid,elongated frame 17 fabricated of a plurality of steel members, includingthree elongated generally vertically disposed beams 19, 21 and 23 whichare symmetrically arranged about a central axis. The beams 19, 21 and 23are joined together and maintained in triangular array by a plurality ofinterconnected cross-braces 25 of various sizes and shapes. The beams 19and 21 are configured with the outer edges having a plurality of teeth27 which are engaged by a geared mounting assembly (schematically shownin FIG. 4) connected to the edge of the platform 13 and adapted toadjustably position the leg and allow it to pivot for inclination of theleg at a desired low angle from the vertical. It will be understood thatany suitable mounting mechanism can be employed to accomplish theintended purposes, including conventional devices. The preciseconstruction of such mechanism does not form part of the presentinvention.

Of more importance, as shown in FIG. 4, a closed can 29 of substantiallycircular cross-section and having a wedge-shaped bottom end 31 adaptedto be driven into the floor is mounted at the bottom end of the frame17. The

interior of the can 29 defines a closed, water-tight chamber.

A manhole assembly 33 provides access to the interior of the can 29 forinspection and repairs when the leg 11 is raised out of the water. Asshown in FIGS. 2 and 3, a valve handle 35 opens and closes a floodingvalve 37 in the can 29 to admit water into the can. The flooding valve37 is opened before the leg 11 is lowered into the water so that the can29 may fill with water as the leg is lowered. When the platform assemblyis to be relocated, the flooding valve 37 is opened upon raising the leg11 out of the water in order to empty the can 29, thereby reducing theweight of the leg for transit purposes.

An air hose 39 is connected between a compressor 40 on the platform 13and a pair of manifold valves 41 located at the top end of the leg 11.The manifold valves 41 are, in turn, connected to a pair of air lines 43which terminate in a pair of check valves 45 inside the can 29. Themanifold valves 41 are normally open so that when the compressor 40 isoperating, pressurized air flows through the air lines 43 and into thecan 29. Since the flooding valve 37 is opened before the leg 11 islowered, the pressurized air is pumped into the can 29 as the leg islowered to establish the desired buoyancy. Should trouble develop in oneof the air lines 43 while the leg 11 is submerged, the manifold valve 41associated with the faulty line may be closed to isolate the faultyline.

A standpipe 47 is mounted in the top of the can 29 with an upper portion49 communicating with the outside of the can and a lower portion 51extending inside the can. With water flowing into the can 29 through theopened flooding valve 37 and pressurized air flowing into the canthrough the check valves 45, the level of water in the can will rise tothe open bottom end of the lower portion 51 of the standpipe 47. If thewater level is below the lower portion 51, the air in the space betweenthe upper surface of the water in the can 29 and the bottom of the lowerportion 51 of the standpipe 47 will escape up the standpipe 47 and outthe upper end thereof, thus permitting the water level in the can 29 torise until it reaches the lower portion 51 at which point it cuts offthe outflow of air and the level of water in the can is stabilized. Inthis manner the position of the sandpipe 47 determines the maximumvolume of air which may be established in the can 29.

In one embodiment of the invention, as shown in FIG. 3, the standpipe 47is secured to the top of the can 29 through a flexible, water-tight seal53 which enables the standpipe to be raised or lowered by a gearingmechanism 55 which is remotely controlled from the platform 13. Byraising and lowering the standpipe 47, the relative maximum quantitiesof Water and air in the can may be varied so as to change the buoyancyof the leg 11 and thereby compensate for different depths and angles ofthe leg in the sea.

The standpipe 47 is mounted adjacent the flat wall of the can 29, asshown in FIG. 2, i.e. the wall adjacent the platform. If the leg isinclined at a small angle with respect to the vertical during thelowering process so that the lower portion extends away from theplatform, a greater volume of air can be established within the can 29,as shown in FIG. 5, in contrast to when the leg 11 is held vertical,since the water level in the leg can 29 is determined by the actuallocation of the bottom end of the lower portion 51 of the standpipe 47.It has been found that the moment in the leg 11 tends to increase as theangle of the leg 11 increases with respect to the vertical. Therefore,the standpipe 47 automatically compensates for changes in the angle ofinclination of the leg 11, while without having to move the standpipe47.

In one embodiment of the invention, as shown in FIG. 3, a plurality oflevel sensing devices 57 are mounted at different levels on the insidesurface of the can 29 and are connected through a conventional servosystem (not shown) to the air compressor 40 on the platform 13. The

level sensing devices 57 provide for the establishment of the waterlevel in the can 29 at any one of a plurality of heights. In thisregard, the standpipe 47 can be lowered to its lowest possible position59' (shown in dotted outline in FIG. 3) near the bottom of the can 29.The water level in the can 29 may then be raised to the desired heightby feeding appropriate information into the servo system. Signals fromthe various water level sensing devices 57 activate the servo system toadjust the pressure of the air pumped into and/ or withdrawn from thecan 29 until the desired water level is reached, whereupon the servosystem null-balances and the air pressure is held constant.

FIG. 4 shows the offshore platform assembly with the legs 11 in a raisedposition for transit. Each leg 11 is joined to the platform 13 by thegeared mounting assembly 61 which includes a pair of pivotably mountedguides 63. Each guide 63 has a slot which receives one of the beams 19,21 and the associated plurality of teeth 27. Gear arrangements mountedwithin the guides 63 engage the teeth 27 and raise or lower the legs 11within the guides 63. Each guide 63 is pivotally mounted to the platform13 at its lower end 65 and pivotably mounted to an incline assembly 67at its upper end 69. The incline assembly 67 establishes the angle ofinclination of the leg 11 by varying the angular position of the guides63.

FIGS. 5 and 6 show a leg 11 in lowered and installed positions,respectively. It will be noted that the leg 11 is inclined at a smallangle with respect to the vertical to give the installed drillingplatform assembly greater rigidity. While each leg 11 experiencesconsiderable bending stress as it is being lowered, the greatest demandsare placed on the legs 11 when they are in the lowered and fullyinclined position of FIG. 5. Accordingly, the volume of air in the can29 should be increased while the leg 11 is in this position, in order tominimize the bending moment experienced by the leg 11. This isautomatically accomplished, since the lower portion 51 of the standpipe47 is positioned adjacent the lower side of the frame 17 when the leg 11is inclined. Thus, the volume of air within the can 29 automaticallyincreases as the angle of inclination of the leg 11 is increased, asshown in FIG. 5, the water level always remaining at the bottom of thelower portion 51 with full air pressure applied to the can.

When the leg 11 is properly positioned on the ocean floor 71, the canbottom 31 and a portion of the can 29 are driven into the fioor to seatthe leg in position. The flow of pressurized air to the inside of thecan 29 is terminated and the can is bled of air, allowing the can tocompletely fill with water and giving the leg 11 increased weight andstability. Thereupon, the platform 13 is jacked up above the surface ofthe water to avoid being buffeted by waves, etc., and the platform isthen ready for use, e.g. drilling operations, etc. When it is desired torelocate the platform assembly, the compressor is turned on,establishing a volume of air in the can 29 and the leg 11 is thencranked up from the floor 71. Air is maintained in the can 29 to reducethe task of raising the leg 11 and to minimize strain on the leg 11.

Accordingly, an improved offshore platform assembly is provided whichincludes one or more legs capable of minimizing the bending forcesthereon through the use of a floodable container adjacent the lower endthereof. The improved construction of the legs allows the apparentweight of the legs to be increased, as for embedding the leg in the seafloor, and also to be decreased to minimize stress on the legs duringinstallation and also during removal from the sea floor. Otheradvantages are as set forth in the foregoing.

Although a specific arrangement of an offshore platform assembly inaccordance with the invention has been described for the purpose ofillustrating the manner in which the invention may be used to advantage,it will be appreciated that the invention is not limited thereto.Accordingly, any and all modifications, variations or equivalentarrangements falling within the scope of the annexed claims should beconsidered to be a part of the invention.

What is claimed is:

1. An offshore platform assembly comprising, in combination, a platformand a plurality of depending legs, means connected to said platform andsaid legs and adapted to adjustably position said legs vertically andlaterally relative to said platform, each of the legs comprising a rigidframe structure having a lower portion capable of being submerged in abody of water, at least one of said legs including a closed chamberdisposed in the submergible portion of the frame structure thereof,means for opening the chamber upon demand when submerged to allow waterto center the chamber, and an adjustable standpipe communicating withthe exterior of the chamber and also having a portion extending to apredetermined level within the chamber and wherein means are connectedto the adjustable standpipe and operable at said platform to vary thelevel of the standpipe within the chamber, whereby the maximum volume ofair which can be established within the chamber is adjustablycontrolled.

2. The offshore assembly of claim 1 wherein the platform is generallyhorizontally extending, wherein each of said legs includes at least oneof said chambers, wherein said means for establishing a volume of aircomprises means for injecting air under pressure into the chamber upondemand and said adjustable standpipe associated with said air injectingmeans and adapted to pass air below a given level in the chamber to theoutside of the chamber, thereby defining a maximum volume of air whichmay be established within the chamber for a given angular position ofthe leg.

3. The offshore assembly of claim 2 wherein said standpipe is positionedadjacent the side of said leg nearest said platform, whereby the volumeof air which can be established in the chamber automatically increaseswith an increase in the angle of inclination of said leg relative tosaid platform.

4. The offshore assembly of claim 3 wherein means are provided in saidchamber, which means are sensitive to the water level in the chamber andare adapted to control the operation of the air injection means so as toestablish a desired level of Water in the chamber.

5. The offshore assembly of claim 4 wherein said assembly is a drillingrig platform assembly and wherein the chamber is located at the lowerend of each leg and has a wedge-shaped bottom adapted to be embedded inthe sea floor.

6. The offshore drilling rig platform assembly of claim 5 wherein thereare three of said legs, each generally triangular in transversecross-section and pivotally mounted in equidistant spaced relation alongthe periphery of said platform.

7. The offshore drilling rig platform assembly of claim 6 wherein eachof said legs is mounted in geared relation on said platform, wherebysaid platform can be raised above the water level when said legs areembeded in the sea floor.

References Cited UNITED STATES PATENTS 1/1952 Harris 61-465 6/1960Quirin 61-465 OTHER REFERENCES JACOB SHAPIRO, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,367,119 February 6, 1968 Chester A. Rybicki It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 2, line 6, "blending" should read bending Column 3, line 13,cancel "by", first occurrence. Column 4, line 4, "repairs" should readrepair line 43, "sandpipe" should read standpipe Column 6 line 17"center should read enter line 61, "embeded" should read embeililedColumn 11 line 52 "unually" should read usua Signed and sealed this 7thday of October 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. JR.

Attesting Officer Commissioner of Patents

